Helical springs have been widely employed to transmit torque between drive and driven members. Such arrangements have been typically employed both as clutches and as brakes.
In the simplest of these prior known arrangements a single, helical spring has been used selectively to transmit torque between two aligned shafts. A representative configuration for this type arrangement is disclosed in U.S. Pat. No. 4,235,133.
Arrangements of the type depicted in U.S. Pat. No. 4,235,133 effect an operative connection between the shafts only when the drive shaft is rotated in one direction. When the drive shaft is rotated in the appropriate direction to transmit torque, the helical spring frictionally grips both the drive and the driven shafts and rotates the driven shaft in response to rotation of the drive shaft. When the drive shaft is rotated in the opposite direction, the helical spring tends to release its frictional engagement of the two, aligned shafts, thus allowing the drive shaft to rotate with respect to the driven shaft. This configuration has been successfully employed for many years, but there are certain definite drawbacks to such an arrangement.
One of the primary drawbacks occurs because the drive and the driven shafts are axially aligned with the clutch spring spanning across the aligned ends of the two shafts. Such an arrangement is highly subject to binding, particularly in the situation where the gap between the aligned shafts is sufficient to allow one coil, or wrap, of the spring to lodge between the opposed ends of the shafts.
One way by which to ameliorate the reliability of this arrangement, and to assure a precise response not only for a driving connection but also for slippage (overrunning), is to provide a tang on each end of the spring so that engagement by the rotating members with one or the other of the tangs will effect a virtually instantaneous response. A representative configuration for a spring clutch which employs tangs is disclosed in U.S. Pat. No. 3,111,822.
Unfortunately, the sharp bend required between the body of the helical spring and the tang subjects such arrangements to serious stress concentrations. Such an arrangement is, therefore, quite conducive to untimely spring failures.
At least one configuration known to the art, which is best exemplified by U.S. Pat. No. 3,242,696, employs stepped, helical springs to effect a driving connection between concentrically disposed drive and driven elements. This is, perhaps, the closest prior art arrangement with which the inventor is aware.